How to define populations
This is one of the most important concepts to agree on before searching
for data. The definition of a population does differ depending on the
species (e.g. the spatial extent for a tree population and a salamander
may not be the same) but here follows some general guidance.
Briefly, ‘population’ refers to a genetically distinct group of
individuals. Within populations the individuals are capable of
interbreeding. Genetic distinction of populations is usually
due to no or very low gene flow (movement of genes e.g. by movement of
offspring) and/or significant adaptive differences, such as due to
occurring in different environments (high/ low altitude, different soil
types, differences in precipitation or temperature). Not every
occurrence is a population! The word ‘subpopulation’ may describe
clusters of organisms across a landscape (including family units) which
are near enough to exchange gene flow. Usually several ‘subpopulations’
are considered together as a population (which can also be called a
‘metapopulation’). When the potential for gene flow is large, e.g.,
viable tree pollen that can travel tens of kilometers, ‘populations’ can
range across large distances, sometimes hundreds of km.
Some published reports will clearly define what a population is based on
the knowledge of biodiversity and taxonomic experts. Some judgment may
be required in interpreting population designation in a report, but in
general the designation of a population by experts should be considered
strongly. In other cases, the reports or database may not clearly
designate population boundaries and will require interpretation. Visual
examination of maps may result (correctly) in ‘merging’ occurrences that
are likely to experience extensive gene flow- ‘likely to experience
extensive gene flow’ may be defined on spatial proximity e.g. close
geographic distance and lack of clear barrier (mountain, fence, etc.).
The distance between subpopulations (or discrete habitat patches where
the species occurs) can be used as a proxy to evaluate to what extent
subpopulations are likely functionally connected. When the edge of a
subpopulation is within a reasonable dispersal distance (for that
particular species, a distance within which some 75-90% of realized
dispersal distances occur- expert discretion is fine here) of another
subpopulation edge, and there are no known physical barriers impeding
dispersal, the subpopulations can be considered as part of the same
metapopulation. Designation of populations should consider human induced
gene flow e.g. genetic rescue, translocations etc. Experts in the
species type (e.g. amphibians) and/or text from the data source can help
identify populations.
Some examples may help. A population may consist of a cluster of
individuals in a discrete location like an island, lake, river catchment
or forest preserve, separate from other discrete locations by some tens
of kilometers. A population may constitute a metapopulation consisting
of subpopulations (ponds, prairies, etc.) that are separate but not very
far (hundreds of meters to several kilometers), thus capable of
exchanging at least 1 migrant (one reproductive adult moving between
patches) on average per generation with each other. Well connected
subpopulations are sufficiently close for the metapopulation to be
called a population, and the population size should be considered the
sum of the subpopulations, which may cover tens or hundreds of
kilometers.
Genetic data can help define populations, but should be used with some
caution. Whether genetic data can detect genetic differences can be a
consequence of the type of genetic marker- new, genomic level markers
can detect very fine genetic divisions, including between
subpopulations. Not every genetic distinction according to DNA markers
is a population. The threshold should still be considered roughly less
than 1 migrant per generation (on average). Older genetic markers such
as chloroplast and mitochondrial DNA sequence are often useful in
identifying highly genetically distinct populations.
Populations of common species with large continuous spatial
distributions that are much larger than the dispersal capacity of a
single individual (e.g. less than 1 migrant per generation across the
extent) are also considered as metapopulations. This applies to common,
abundant species. Many populations of common species will be either
nearly continuous or occur over very large areas. When very large
continuous populations exist, e.g. hundreds of kilometers, then somewhat
arbitrary population boundaries should be instituted, based on
ecological changes such asecoregions.
Metapopulations should represent stable spatial and temporal units. Many
species have ephemeral subpopulations in dynamic source-sink
metapopulations- a sink is a spatial location receiving high immigration
from adjacent areas and which would not persist on its own without
immigration. Sinks are not distinct populations. The conglomerate of
connected subpopulations that should be evaluated as a population.
For freshwater fish, the riverscape structure can help define
populations or units that can be assimilated to
populations/metapopulations with a little GIS work. Individuals
inhabiting lakes can be considered as populations, especially for lakes
that are disconnected from the hydrographical network. Riverscape
(meta)populations can also be defined through their level of
connection/disconnection e.g. belonging to different river basins,
hydrographical systems, river stretches separated by huge dams, etc.
For trees, the pollination mode and commonness is important. Trees which
are wind-pollinated can have continuous populations extend over tens of
kilometers, and farther. Trees which are insect pollinated generally but
not always, have less gene flow. For common trees, a distinct population
may not be easily apparent (e.g. trees that extend across much of a
continent in a continuous fashion). In such cases, a “population” may
be considered at approximately the level of a state, country, or
ecoregion (hundreds of kilometers across).